Optical: systems and elements – Single channel simultaneously to or from plural channels – By refraction at beam splitting or combining surface
Reexamination Certificate
1999-12-29
2001-01-09
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By refraction at beam splitting or combining surface
C359S634000
Reexamination Certificate
active
06172815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a projection display apparatus that separates light emitted from a light source into color beams of at least two colors, modulates these color beams via light valves formed of liquid crystal panels according to image information, synthesizes the modulated color beams again, and projects and displays the synthesized beams onto a projection plane via a projection optical system. More particularly, the present invention relates to a dichroic prism capable of correcting relative displacements of projection pixels of the colors caused by chromatic aberration of magnification.
2. Description of Related Art
A well-known type of projection display apparatus includes a light source, a color separation device that separates a light beam emitted from the light source into color beams of three colors, three light valves for modulating the separated color beams according to image information, a dichroic prism serving as a color synthesizing device that synthesizes the color beams modulated by the three light valves, and a projection optical system for enlarging and projecting the beams synthesized by the dichroic prism onto a projection plane.
The dichroic prism is disclosed in, for example, Japanese Unexamined Patent Publication No. 7-294845. As shown in
FIG. 1
, a dichroic prism
1000
disclosed in this publication is shaped like a quadrangular prism as a whole by bonding four rectangular prisms
11
,
12
,
13
, and
14
having a cross-section shaped like a rectangular isosceles triangle. In this dichroic prism
1000
, red reflecting dichroic planes
1500
and
1500
and blue reflecting dichroic planes
1600
and
1600
diagonally extend along the junction surfaces of the prisms
11
-
14
so that they form a flat red reflecting dichroic plane and a flat blue reflecting dichroic plane, respectively. A layer of an adhesive
1800
, which is formed between the junction surfaces to join the rectangular prisms
11
-
14
, is 10 &mgr;m to 20 &mgr;m thick.
Such a dichroic prism
1000
is used as a color synthesizing device in a projection display apparatus. In this case, color beams R, G, and B from liquid crystal light valves
925
R,
925
G, and
925
B, which are placed to oppose three sides of the dichroic prism
1000
, are synthesized by the pair of dichroic planes
1500
and
1600
, and the synthesized beams are emitted from the other side of the dichroic prism
1000
toward a projection optical system.
Since the influence of chromatic aberration of magnification on images of the colors, which are projected and displayed via the projection optical system, increases with the distance from the center, displacements of the projection images of the colors become large. For example, as shown in
FIG. 12
, the areas of color projection images projected on a projection plane
100
, a blue projection image B′, a green projection image G′, and a red projection image R′, increase in this order. In terms of projection pixels, projection pixels b, g, and r of the colors are not in complete alignment, as shown in FIGS.
13
(A)-
13
(C), and there is a pixel displacement d
1
. While the pixel displacement d
1
hardly affects display quality when it is small (d
1
≦d), it degrades the quality of a projection image when it exceeds the permitted limit (d≦d
1
=d
2
). The sizes of the color projection images R′, G′, and B′ vary with the material of the projection optical system. The red projection image R′ is not always the largest, and the areas of the projection images of the colors relatively differ from one another.
In the industry associated with projection display apparatuses, efforts have been made to increase the definition of a light valve formed of a liquid crystal panel and the like by increasing the number of pixels of the light valve. There has been a tendency to reduce the area of each pixel with this increase of definition, which lowers the permitted limit of the above-described pixel displacement d
1
and decreases the tolerance for chromatic aberration of magnification. Therefore, there is a need to reduce the influence of chromatic aberration of magnification on a projection image by correcting chromatic aberration of magnification, in order to obtain a high-resolution and high-quality projection image.
SUMMARY OF THE INVENTION
In view of the above problems, an object of the present invention is to provide a dichroic prism that can reduce displacements of projection pixels of colors caused by chromatic aberration of magnification. Another object of the present invention is to provide a projection display apparatus in which the dichroic prism is incorporated.
In order to achieve the above objects, the present invention provides a dichroic prism that is formed by joining four prisms of a triangular cross-section, and is provided with first and second dichroic planes intersecting so as to form approximately an X shape along joint surfaces of the prisms and having different reflectance characteristics, wherein at least the first dichroic plane of the first and second dichroic planes is either concave or convex.
In such a dichroic prism of the present invention, the reflection state of a color beam to be reflected from the first dichroic plane varies depending on the degree of concavity or convexity of the first dichroic plane. Therefore, it is possible to either enlarge or reduce the color beam. Since the magnification of a projection image of a desired color beam can be adjusted through the use of the dichroic prism of the present invention as a color synthesizing device in the projection display apparatus, the areas of projection images of colors can be made almost equal. That is, it is possible to correct chromatic aberration of magnification, and to thereby reduce relative displacements of projection pixels of the color beams. Accordingly, the use of the dichroic prism of the present invention as a color synthesizing device in the projection display apparatus makes it possible to prevent degradation of a projection image due to chromatic aberration of magnification.
There has been a trend in the industry associated with projection display apparatuses to increase the definition of the light valve in order to obtain a projection image with higher resolution. In this case, the permitted limit of pixel displacements is lowered, and the tolerance for chromatic aberration of magnification is decreased. Since the dichroic prism of the present invention can, as mentioned above, correct chromatic aberration of magnification and thereby reduce relative displacements of pixels of the colors, it is particularly suitable for a color synthesizing device to be employed in a projection display apparatus that projects a high-resolution and high-quality image through the use of a high-definition light valve.
It is preferable that the first dichroic plane be inclined symmetrically with respect to a line of intersection of the first and second dichroic planes. Although the first dichroic plane may be, for example, curved, since it is necessary in this case to manufacture the prisms focusing on the radius of curvature and the like, manufacturing of the dichroic prism is expected to be difficult. Accordingly, when the first dichroic plane is formed as described above, it is possible to facilitate the manufacturing and to prevent a steep rise in manufacturing cost.
Rectangular prisms having a right vertical angle may be used as the prisms of triangular cross-section. In this case, the first dichroic plane can be made concave or convex by adjusting the thickness of an adhesive layer for joining these rectangular prisms. Such use of the rectangular prisms allows the rectangular prisms to constitute the conventional dichroic mirror to be used unchanged.
When at least one of the four prisms is replaced with a prism having an acute or obtuse vertical angle, it is possible to naturally make the first dichroic plane concave or convex without intentionally adjusting the thickness of the
Hashizume Toshiaki
Yajima Akitaka
Mack Ricky
Oliff & Berridg,e PLC
Seiko Epson Corporation
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